As is well known, the spinal cord is part of the central nervous system and is connected to the brain. From the brain, the cord runs down the back and is surrounded by and protected by the bony vertebral column. The cord is surrounded by fluid called “cerebral spinal fluid,” that acts as cushion to protect the delicate nervous tissues. The cord itself consists of millions of nerve fibers that transmit information to and from the limbs, trunk and organs of the body. Nerves which are typically called the spinal nerves or nerve roots come off the spinal cord and pass out between the vertebrae to carry information from the spinal cord to the rest of the body.
Cervical nerves, which are nerves in the neck, supply movement and feeling to the neck and upper trunk; thoracic nerves, which are nerves in the upper back, supply the trunk and abdomen; and lumbar and sacral nerves, which are nerves from the lower back, supply the legs, the bladder, bowel and sexual organs. The spinal nerves carry information from different levels (segments) in the spinal cord and the nerves and segments in the spinal cord are numbered in the same way. Accordingly, the cervical nerves and spinal cord segments are identified as C1-C8, the thoracic and lumbar are T1-T12 and L1-L5 and the sacral are S1-S5.
Metastatic disease occurs when cancer from one site in the body spreads to another area. The spine is a common area for the spread of cancer and can cause a variety of neurological and bone related symptoms. A number of studies have found that between about 30% to about 70% of cancer patients had spinal metastases when autopsies were conducted. Metastases to the spinal column and spinal cord commonly originate from cancers occurring in the lung, breast, gastrointestinal tract, prostrate and lymphomas. Metastases to the spine are primarily treated in order to control pain, maintain or restore stability, and to preserve or restore function as much as possible.
Some of the different treatments for treating the metastases include radiotherapy and surgical radical resection. In addition, a decompressive laminectomy can be performed in which a portion of the vertebral bone called the lamina is removed. There are many variations of laminectomy and typically, the traditional procedure excises much more than just the entire lamina in that the entire posterior backbone is removed, along with overlying ligaments and muscles. The lamina is a posterior arch of the vertebral bone lying between the spinous process, which juts out in the midline, and the more lateral of each vertebra. The pair of lamina, along with the spinous process, makes up the posterior wall of the bony spinal canal. The lamina is typically removed in order to allow the surgeon access to deeper tissues inside the spinal canal, to perform a direct decompression for dorsally located lesions, or an indirect decompression for lesions that are in front of the spinal cord (allowing the spinal cord to fall away from the lesion).
Another approach surgeons use to treat spinal metastases is a transpedicular approach, which is popular when the tumor involves the dorsal or lateral aspect of the vertebral body, especially when the disease extends into the pedicle and associated dorsal elements. Facetectomy coupled with pedicle resection allows access into the vertebral body. Often coupled with rigid instrumentation above and below, this procedure provides an excellent surgical result.
Whereas primary prostrate tumors are treated with much higher doses of radiation (either as fractionated external beam radiation, say, with 85 Gy or a combination of directly implanted radioactive brachytherapy seeds, say, providing 100 Gy and local fractionated radiation for 45 Gy), spinal metastases are currently treated by doses on the order of 30-55 Gy of fractionated external beam radiation, which is less effective than the higher doses but is limited primarily because of concern about radiation induced myelopathy (damage to the spinal cord) seen with higher doses of radiation.
One other deficiency of traditional radiation treatment to spinal pathology is the problem of primary or secondary epidural tumors in the spine recurring after initial resection and external fractionated radiotherapy. Given the confines of the surgical site, adequate local excision with wide margins is difficult to perform. In view of the foregoing, postoperative irradiation is a valuable adjuvant treatment; however, irradiation of the spine has specific limitations that often result in inadequate tumor dose. It has been suggested that shielding of the spinal cord may permit greater radiation doses to be safely delivered by brachytherapy to recurrent tumors. Brachytherapy, also referred to as sealed source radiotherapy or endocurietherapy, is a form of radiotherapy where a radioactive source is placed inside or next to the area requiring treatment. The sealed radioactive source is also commonly referred to as a “seed” and/or “source.” However, the shielding that has been proposed, such as in Hamilton, Lula et al., Int. J. Rad One Biol Physics 32(2) 507-511, 1995, has a number of disadvantages that limits its use and effectiveness. In particular, the shield was made of a simple foil material and was used in the epidural plane and did not have a particular contour that would assist in placement and the effectiveness of the shield when placed between the spinal dura and spinal vertebral body. In addition, this type of foil shield had other limitations in that the shield was placed on the dorsal surface of the cord, whereas the roots prevented placement of the shield ventral to the cord. The shield was also used after a multiply-recurrent tumor was re-resected using a large standard “open” technique in only one very young patient in what seemed a less than rigorous manner. Yet another disadvantage of the foil technique is that the foil scatters back the radiation and this scattering can be random.
There is no significant current utilization of implanted radioactive seeds and there are only scattered references to intraoperative doses of brachytherapy, none involving the concomitant use of seeds and bone cement.
There is consequently a need for a shield or the like that is specifically contoured for placement between the spinal dura and the spinal vertebral body and be able to be implanted through either an open or a minimally invasive technique in order to shield the spine from all sources of radiation across the electro-magnetic spectrum.